Field of the Invention
The present invention relates generally to multi-piece golf balls having a core and surrounding cover. In one embodiment, the core has a dual-layered structure, wherein the inner core (center) is made from a foam or metal-containing composition, or has a hollow shell construction, and the outer core layer is made of a thermoplastic composition. Preferably, the thermoplastic composition comprises an ethylene acid copolymer ionomer and plasticizer.
Brief Review of the Related Art
Multi-layered, golf balls are used today by recreational and professional golfers. In general, these golf balls contain an inner core protected by a cover. The core acts as the primary engine for the ball and the cover protects the core and helps provide the ball with durability and wear-resistance. The core and cover may be single or multi-layered. For example, three-piece golf balls having an inner core, inner cover layer, and outer cover layer are popular. In other instances, golfers will use a four-piece ball containing a dual-core (inner core and surrounding outer-core layer) and dual-cover (inner cover layer and surrounding outer cover layer). Intermediate layer(s) may be disposed between the core and cover layers to impart various properties. Thus, five-piece and even six-piece balls can be made. Normally, the core layers are made of a natural or synthetic rubber material or an ionomer polymer. These ionomer polymers are typically copolymers of ethylene and methacrylic acid or acrylic acid that are partially or fully neutralized. In particular, highly neutralized polymer (HNP) compositions may be used to form a core layer. Metal ions such as sodium, lithium, zinc, and magnesium are commonly used to neutralize the acid groups in the acid copolymer.
Such ethylene acid copolymer ionomer resins generally have good durability, cut-resistance, and toughness. These ionomers may be used to make cover, intermediate, and core layers for the golf ball. When used as a core material, the ionomer resin helps impart a higher initial velocity to the golf ball.
As noted above, in recent years, three-piece, four-piece, and even five-piece balls have become more popular. New manufacturing technologies, lower material costs, and desirable ball playing performance properties have contributed to these multi-piece balls becoming more popular. Many golf balls used today have multi-layered cores comprising an inner core and at least one surrounding outer core layer. For example, the inner core may be made of a relatively soft and resilient material, while the outer core may be made of a harder and more rigid material. The “dual-core” sub-assembly is encapsulated by a cover of at least one layer to provide a final ball assembly. Different materials can be used to manufacture the core sub-assembly including traditional materials such as polybutadiene rubber and ethylene acid copolymer ionomers. In other instances, non-traditional materials such as metal and foam are used to form at least one of the core layers.
For example, Nesbitt and Binette, U.S. Pat. No. 6,277,034 disclose a multi-piece golf ball containing a spherical metal core component having a specific gravity of about 1.5 to about 19.4; and an outer core layer having a specific gravity of less than 1.2. The metal core preferably contains a metal selected from steel, titanium, brass, lead, tungsten, molybdenum, copper, nickel, iron, and combinations thereof. Polybutadiene rubber compositions containing metallic powders can be used to form the core.
Sullivan, U.S. Pat. No. 6,494,795 discloses a golf ball comprising an inner core having a specific gravity of greater than 1.8 encased within a first mantle. The core may be made from a high density metal or from metal powder encased in a polymeric binder. High density metals such as steel, tungsten, lead, brass, bronze, copper, nickel, molybdenum, or alloys may be used. The mantle layer may be made from a thermoset or thermoplastic material such as epoxy, urethane, polyester, or polyurethane, or polyurea.
Sullivan, U.S. Pat. No. 6,692,380 discloses a golf ball comprising an inner core having a specific gravity of at least 3, a diameter of about 0.40 to about 0.60 inches and preferably comprises a polymeric matrix of polyurethane, polyurea, or blends thereof. The outer core may be made from a polybutadiene rubber. The specific gravity of the compositions may be adjusted by adding fillers such as metal powder, metal alloy powder, metal oxide, metal stearates, particulates, and carbonaceous material.
Core structures having a foamed layer also have been generally disclosed in the patent literature. For example, Sullivan and Ladd, U.S. Pat. No. 6,688,991 discloses a golf ball containing a low specific gravity core and optional intermediate layer. This sub-assembly is encased within a high specific gravity cover with a Shore D hardness of 40 to 80. The core is preferably made from a highly neutralized thermoplastic polymer such as ethylene acid copolymer which has been foamed. The cover preferably has high specific gravity fillers dispersed therein.
Nesbitt, U.S. Pat. No. 6,767,294 discloses a golf ball comprising: i) a pressurized foamed inner center formed from a thermoset material, a thermoplastic material, or combinations thereof, a blowing agent and a cross-linking agent and, ii) an outer core layer formed from a second thermoset material, a thermoplastic material, or combinations thereof. Additionally, a barrier resin or film can be applied over the outer core layer to reduce the diffusion of the internal gas and pressure from the nucleus (center and outer core layer).
Regarding hollow core structures, Yoshida et al., U.S. Pat. No. 6,315,683 is generally directed to an over-sized (greater than 1.70 inches) hollow solid golf ball where the hollow core is contained in a thermoset rubber layer and covered with a single ionomer cover. Also, Nakamura et al., U.S. Pat. No. 8,262,508 generally describes a golf ball having a hollow center, a mid-layer, an inner cover, and an outer cover. The hollow center and mid-layer are both formed from a thermoset rubber composition,
Although some of the above-described compositions may be somewhat effective for making certain components and layers in a golf ball, there is still a need for new compositions that can impart high performance properties to the ball. Particularly, there is a continuing need for improved core constructions in golf balls. In particular, two and three-layered core constructions are needed, wherein the core structure has good toughness and provides the ball with high resiliency. At the same time, the core assembly should not be excessively hard and stiff so that properties such as feel, softness, and spin control are sacrificed. The present invention provides golf balls having an optimum combination of properties.